Hypoxia can lead to solid tumor aggressiveness by driving multiple signaling pathways. Long non-coding RNAs respond to several extrinsic stimuli, causing changes in cancer cells by participating in multiple steps of gene expression. However, genomic profiling of long non-coding RNAs regulated by oxygen in breast cancer remained unclear. Therefore, the aims of this study were to identify oxygen-responsive long non-coding RNAs in breast cancer cells, and to delineate their regulatory mechanisms. The expression profiling of long non-coding RNAs in breast cancer cells growing under normoxic, hypoxic, and re-oxygenated conditions was examined using next-generation sequencing technology. Four hundred and seventy-two lncRNAs oxygen-responsive lncRNAs were identified. After examining the top three differentially expressed lncRNAs in hypoxia, we selected N-Myc Downstream Regulated Gene 1-Overlapping 1 (NDRG1-OT1) for further study, especially the most responsive isoform, NDRG1-OT1_v4. We overexpressed NDRG1-OT1_v4 under normoxia and performed microarray analysis to identify 108 NDRG1-OT1_v4 regulated genes and their functions. Among these genes, we found that both NDRG1 mRNA expression and NDRG1 protein levels were inhibited by NDRG1-OT1_v4. Finally, we used co-immunoprecipitation to show that NDRG1-OT1_v4 destabilizes NDRG1 by promoting ubiquitin-mediated proteolysis. Our findings reveal a new type of epigenetic regulation of NDRG1 by NDRG1-OT1_v4 in breast cancer cells.

Hypoxia plays a crucial role in the aggressiveness of solid tumors by driving multiple signaling pathways. Recently, long non-coding RNA (lncRNA) has been reported to promote or inhibit tumor aggressiveness by regulating gene expression. Previous studies in our laboratory found that the lncRNA NDRG1-OT1 is significantly up-regulated under hypoxia and inhibits its target gene NDRG1 at both the mRNA and protein levels. At the protein level, NDRG1-OT1 increases NDRG1 degradation via ubiquitin-mediated proteolysis. However, the repressive mechanism of NDRG1 at the RNA level is still unknown. Therefore, the purpose of this study was to study how NDRG1-OT1 transcriptionally regulates its target gene NDRG1. Luciferase reporter assays showed that NDRG1-OT1 decreased NDRG1 promoter activities. Mass spectrometry, bioinformatics tools, genetic manipulation, and immunoblotting were used to identify the interacting proteins. Surprisingly, different fragments of NDRG1-OT1 had opposite effects on NDRG1. The first quarter fragment (1-149 nt) of NDRG1-OT 1 had no effect on the NDRG1 promoter; the second quarter fragment (150-263 nt) repressed NDRG1 by increasing the binding affinity of HNRNPA1; the third quarter fragment (264-392 nt) improved NDRG1 promoter activity by recruiting HIF-1α; the fourth quarter fragment (393-508 nt) down-regulated NDRG1 promoter activity via down-regulation of KHSRP under hypoxia. In summary, we have found a novel mechanism by which different fragments of the same lncRNA can cause opposite effects within the same target gene.

Hypoxia is a crucial factor in aggressiveness of solid tumor by driving multiple signaling pathways. Recent researchers indicated that long non-coding RNA (lncRNA) could promote or inhibit tumor aggressiveness by regulating gene expression. Previous studies in our laboratory found the expression of lncRNA NDRG1-OT1_v4 significantly increased under hypoxia by next-generation sequencing (NGS). Moreover, it was discovered that the NDRG1-OT1_v4 inhibited NDRG1 at both mRNA and protein levels of NDRG1. At the protein level, NDRG1-OT1_v4 improved NDRG1 degradation via ubiquitin-mediated proteolysis pathway. However, the repressive mechanism of NDRG1 at RNA level was still unknown. In this studies, we found that NDRG1-OT1_v4 decreased the NDRG1 promoter activities when we overexpressed NDRG1-OT1_v4 under hypoxia. The fragment (150-263 nt) of NDRG1-OT1_v4 repressed NDRG1 promoter activity significantly by increasing the binding affinity of hnRNPA1. On the other hand, another fragment (264-392 nt) of NDRG1-OT1_v4 improved NDRG1 promoter activity by recruiting HIF-1 alpha. In conclusion, we found a novel mechanism that different fragments of same lncRNA could cause opposite effects on the identical target gene. Keywords: Hypoxia, LncRNA, NDRG1-OT1_v4, NDRG1, HIF-1 alpha, hnRNPA1 Citation Format: Chingching Yeh, Mong-Hsun Tsai, Eric Chuang, Liang-Chuan Lai. Different effects of lncRNA NDRG1-OT1_v4 fragments on regulating NDRG1 transcription in breast cancer cells under hypoxia [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 1496.